Early InstallWe will be unlocking the game's installer at 8:01 a.m. PDT on May 14. This will allow you to begin installing the game, troubleshoot any issues you run into, download and install the launch day 1.0.1 patch, and be ready to play as soon as the servers come online.

No, the "Unexpected Decrytion Error" message isn't normal and indicates one of two things. Either you've moved the Diablo III Setup.exe file out of the folder it's supposed to be in before running it (unlikely) or the download didn't finish, leaving one or more .part files instead of the completed files. In particular, I'd imagine there isn't an "Installer Tome 1.MPQE" there yet.

Re-run the downloader specifying the same location you used before and it will check the files it already has and then download what it still needs. When it appears to be done, even if it says it's finished, let it continue. If it runs to completion, it should launch the setup file itself. That's when you get the message about it not being released yet.